Regulation of the dynamic localization of the rat Bsep gene-encoded bile salt export pump by anisoosmolarity

Canalicular transport via the bile salt export pump (Bsep) represents the r ate-controlling step in taurocholate excretion, whose capacity is under osm otic control. The shortterm effects of anisoosmolarity and Ca2+-withdrawal on the localization of Bsep and the tight junction proteins Zo-1 and occlud in were studied in perfused rat liver by immunohistochemistry, confocal mic roscopy, and densitometry, Under normoosmotic conditions, Bsep was found in the canalicular membrane and showed a punctate intracellular localization. Hypoosmolarity resulted in the translocation of intracellular Bsep to the canalicular membrane, whereas hyperosmolarity induced a retrieval of Bsep, Following hyperosmolar retrieval of Bsep and multidrug resistance protein 2 (Mrp2) from the canalicular membrane, in the putative intracellular vesicl es Bsep and Mrp2 colocalized in 15% of these vesicles, whereas 85% stained either positive for Bsep (61%) or Mrp2 (24%), Anisotonicity had no effect o n the linear staining patterns of occludin and Zo-1, indicating no increase in paracellular permeability, Omission of calcium produced cholestasis cha racterized by a disruption of occludin, whereas the localization of Zo-1, B sep, and Mrp2 remained unaffected. It is concluded (1) that hyperosmolarity induces retrieval of Bsep from the canalicular membrane, which correlates to cholestasis, Hypoosmolarity leads to choleresis accompanied by a rapid r ecruitment of intracellular Bsep to the canalicular membrane. (2) Bsep- and Mrp2-specific vesicles participate in the short-term osmoregulation of can alicular secretion, however, a cause-effect relationship between bile salt excretion and transporter localization remains to be established. (3) Ca2+- depletion induces cholestasis by disruption of occludin-determined tight ju nctional permeability, whereas internalization of canalicular transporters play a minor role.